The present disclosure relates to image forming apparatuses to which the two-component development technique is applied.
A conventionally known image forming apparatus for forming an image on a sheet is provided with a photosensitive drum (image bearing member), a developing device, and a transfer member. In such an image forming apparatus, an electrostatic latent image formed on the photosensitive drum is transformed into a visible image by the developing device, so that a toner image is formed on the photosensitive drum. Thereafter, the toner image is transferred to a sheet by the transfer member. As a development technique of transforming an electrostatic latent image to a visible image, i.e. a toner image, the two-component development technique of using a developer containing toner and carrier is known.
In the two-component development, observed is the phenomenon that the developer degrades due to influences such as the number of printed copies, environmental variations, the printing mode (the number of successively printed copies per job), and the page coverage, so that the amount of electrostatic charge of toner is altered. This leads to problems such as a reduction in image density, the occurrence of toner fog, and the scattering of toner. In order to address such problems, some techniques have conventionally been employed in which a change in the amount of electrostatic charge of toner is predicted from the number of printed copies, environmental variations, the printing mode, the page coverage, etc., and the toner density, the development bias, the surface potential of the photoreceptor, the rotational speed of the development roller, the output of the suction fan for collecting scattered toner, etc., are adjusted so as to reduce or inhibit the reduction of image density, the exacerbation of toner fog, and the exacerbation of toner scattering.
However, these techniques are only based on the combination of predictions under respective conditions, i.e. the number of printed copies, environmental variations, the printing mode, and the page coverage. Therefore, if these conditions are changed together, it is difficult to accurately predict the amount of electrostatic charge of toner.
Therefore, some techniques of accurately predicting the amount of electrostatic charge of toner have conventionally been proposed. For example, while a development bias is being applied to the development roller bearing the developer, the current value of a current (hereinafter referred to as a “development current”) flowing between the photosensitive drum and the development roller is measured. It is assumed that the measured current value of the development current is equal to the amount of electric charge of toner moved from the development roller to the photosensitive drum. The amount of the toner is calculated from the result of measurement of the density of a developed toner image. The amount of electrostatic charge on the toner is calculated from the amount of electric charge of the toner and the amount of the toner.